docs: def tag(tags):
          """Returns a function that adds the words in `tags` as class tags to this class."""
          def dec(cls):
              """Add the words in `tags` as class tags to this class."""
              # Add in this class's tags
              cls._class_tags.update(tags.replace(",", " ").split())  # pylint: disable=protected-access
              return cls
          return dec